Generally, making very high rpm bearings has long posed technical challenges. Mechanical bearings have a high failure rate when operated at speeds over about 15,000 rpm. Improvements have generally included providing lubricants and smoother bearing races, and constructing the bearing from ever-stronger materials. Nonetheless, operating a mechanical bearing faster than about 70,000 rpm can be viewed as risky.
Accordingly, some have tried making magnetic bearings. One kind of magnetic bearing is an active system in which current from a power source is regulated to cause a stationary member to magnetically interact with one or more magnets on a moving member and thereby influence the position of the moving member. For example, in a rotary embodiment, a coil in a housing can be powered to magnetically pull a rotating rotor in closer. Two coils acting in tandem can sequentially pull such a rotor into a central position if the excitation frequency of the coils is sufficiently higher than the characteristic system inertial response time.
Another kind of magnetic bearing involves null flux coils. A "null flux coil" is a closed electric circuit, typically but not necessarily shaped like a FIG. "8, " wherein the sum of the fluxes produced by the circuit is zero. That is, no voltage is induced in the circuit when the flux is changing with time.
Certain uses of null flux coils have been proposed for some time. For example, U.S. Pat. No. 1,020,943 Levitating Transmitting Apparatus (Bachelet) issued Mar. 19, 1912, proposes using null flux coils in magnetic suspension systems for moving vehicles.
As to rotational applications in which current in a coil is increased or decreased actively to move a ferromagnetic disk closer or farther from a coil respectively, consider the following examples: U.S. Pat. No. 3,890,019 Magnetic Bearings (Boden), issued Jun. 17, 1975, and U.S. Pat. No. 4,065,189 Magnetically Suspended Angular Momentum Wheel (Sikorra), issued Dec. 27, 1977.
As to primarily translational applications, a null flux magnetic vehicle suspension system is mentioned in U.S. Pat. No. 3,470,828 Electromagnetic Inductive Suspension and Stabilization System For A Ground Vehicle (Powell), issued Oct. 7, 1969. Field sources comprised of two superconducting current loops are mounted on a vehicle. The two loops carry current in opposite directions, both directing flux at a central null flux coil. Track-based null flux coils made of electrically conductive, nonmagnetic material are positioned so that the superconducting current loops induce a current in null flux coils whenever the coils deviate from their equilibrium position.
U.S. Pat. No. 3,834,317 Magnetic Moving Vehicle Suspension (Miericke), issued Sep. 10, 1974, discusses a similar geometry using two sets of magnet coil loops, the loops having the same axis, and the loops each carrying current in opposite directions. Instead of using null flux coils, conducting laminated aluminum plates sandwiched between the loops, as well as at the center of the fringe region between the coil pairs, are used to produce stability and lift. The coils thus set up a central opposing field which acts to center a plate conductor. In this geometry a central fringe region field between one coil pair and another similar pair is used to produce lateral stability as well.
With the approaches of both Powell and Miericke, the magnetic field source loops carry current in opposite directions. These magnetic loops produce fields that are in repulsion, which creates a flux pattern having a gradient and no net flux passing through a plane located centrally between the loops. A combination of two such magnetic loops (energized coils), permanent magnets, electromagnets, and their equivalents will herein be referred to as repulsive magnetic field sources.
In contrast to the approach disclosed by Powell, U.S. Pat. No. 4,913,059 Levitation-Propulsion Mechanism For Inductive Repulsion-Type Magnetically Levitated Railway (Funji), issued Apr. 3, 1990, and U.S. Pat. No. 4,779,538 Levitation, Propulsion and Guidance Mechanism For Inductive Repulsion-Type Magnetically Levitated Railway (Fujiwara), issued Oct. 25, 1988, disclose magnet field loops excited to drive flux through the null flux coils. This creates a flux pattern passing in a direction, e.g., through a plane located centrally between the loops. This flux pattern can be produced with only one magnetic pole of a permanent magnet, magnetic loop (energized coil), electromagnet, or the like; though, two poles in field attraction are preferable. Such a source will herein be referred to as a transverse magnetic field source.
Restoring forces are generated because one of the two loops comprising the null flux coil links more flux if the loop is misaligned. Current induced commensurate with this additional linkage creates a restoring force to equalize the flux linkage of the two null flux coil loops. In the geometry of the Funji and the Fujiwara patents, the restoring force created by transverse magnetic field sources gives the vehicle lift. The teachings of these two patents are similar, as they involve a null flux loop on either side of the vehicle to produce lateral stabilization. Fujiwara also mentions connecting the two loops in series for more efficient lateral stabilization. In either case, though, the lateral stabilization is realized by the spatial decay of a B field away from primary superconducting magnet loops.
Drawbacks to the prior active bearing approaches tend to involve the need to detect how close the moving member is to the stationary member, and the use of circuitry responsive to the detection in order to regulate current to increase or decrease the pull on the magnet(s) on the moving member. It is difficult to detect and then regulate the current quick enough, for example, to keep a rotor stable at very high rpms, e,g., over 90,000 rpm.
Another drawback is that, if there is a power failure for an active system, the consequences can be catastrophic: control is lost for the bearing, which is spinning at a very high rpm, and from this, the support provided by the bearing for a component moving at the same speed in a machine is also lost. Unsupported components traveling at very high speeds portend catastrophe.
Other drawbacks of an active magnetic bearing system include the cost for such systems and the lack of design flexibility that result from the need to provide power control at a very high frequency.
Design flexibility is a particular concern for rotational applications. In the above-cited patents, those which involve passive null flux coil systems are for linear applications, and it is not apparent how any of these approaches might be used in a rotational geometry.
Efficiency is another concern in that they all use at least two null flux coils to realize the levitation and lift forces. Most of these systems use separate coils to realize each of these forces. Fujiwara is perhaps the most efficient in that it uses two null flux coils connected in series, but the cost for such an approach must be significant. So far as is known, no prior art system has been discovered in which a single passive null flux coil and two different types of magnetic field sources--transverse and repulsive--realize both levitation and lift forces.